Use the Fluid-Film Properties node to set the wall properties, base properties, fluid properties, and the film flow model.

In the Thin-Film Flow (tff) 2D and 3D interfaces, the Reference normal orientation setting allows you to reverse the direction of the reference surface normal used by the physics interface. By default this is set to Same direction as geometry normal. To reverse the direction, select Opposite direction to geometry normal.

In the Thin-Film Flow, Domain (tff) interface, the Reference surface normal orientation setting allows you to set the direction of the reference normal so that it either points out of or into the 2D plane. By default this is set to Reference surface normal points out of plane, such that the reference surface normal is given by the z direction {0,0,1}. Selecting Reference surface normal points into plane sets the reference surface normal to {0,0,-1}.

Enter a value or expression for the Height of wall above reference plane hw1 (SI unit: m). The default is 10 μm.

By default the Additional wall displacement is User defined. Enter values or expressions for uw (SI unit: m). The defaults are 0 m. Alternatively select a feature input (defined by a separate physics interface) or None from the list.

When using a feature input to define an additional displacement based on the movement of a structure, refer to Figure 9-1 to decide whether the additional displacement should be added to the wall or the base.

If the reference normal points into the structure for which the displacement is provided, the structure is the base and its displacement should be added to the additional base displacement.To determine the reference normal orientation before fully solving the model, click Show Default Solver () in the Study toolbar (or right-click the Study 1 node and select Show Default Solver). Then expand the Study 1>Solver Configurations>Solver 1 node, right-click the Dependent variables subnode and select Compute to Selected.

You can then add a 2D or 3D plot group with an Arrow Surface plot (in 3D) () or Arrow Line plot (in 2D) (), and use the Replace Expression () button to plot the Reference surface normal.

Note that the orientation of the reference surface normal can be changed using the Reference normal orientation setting in the Reference Surface Properties section.

Select a Wall velocity vw (SI unit: m/s) — None (the default), Calculate from wall displacement, or User defined. For User defined enter values or expressions for the components of vw (SI unit: m/s). The defaults are 0 m/s.

See Wall Properties above for details to help you decide whether the additional displacement should be added to the wall or the base.

Enter a value or expression for the Height of base below reference plane hb1 (SI unit: m). The default is 0 m.

By default Additional base displacement is User defined. Enter values or expressions for ub (SI unit: m). The defaults are 0 m. Or select None from the list.

Select a Base velocity vb (SI unit: m/s) — None (the default), Calculate from base displacement, or User defined. For User defined enter values or expressions for the components of vb (SI unit: m/s). The defaults are 0 m/s.

The default Dynamic viscosity μ (SI unit: Pa·s) is taken From material. For User defined enter a different value or expression. The default is 0 Pa·s.

With the default options, the Density ρ (SI unit: kg/m3) is taken From material. For User defined enter a different value or expression. The default is 0 kg/m3.

When Gas (modified Reynolds equation) is selected in the Physical Model section, the density is determined by the ideal gas law.

When Liquid with cavitation is selected in the Physical Model section, the density depends on the pressure as ρ = ρc exp(βpfilm) where pfilm is the fluid pressure, ρc is the density at the cavitation pressure, and β is the fluid compressibility (SI unit: 1/Pa). In this case enter the values for the Density at cavitation pressure ρc (SI unit: Pa) and the Compressibility β (SI unit: 1/Pa).

Select a Film flow model — No-slip walls (the default), Slip at walls, User defined-relative flow function, or User defined-general. The film flow model is used to compute the mean fluid velocity as a function of the pressure gradient, the wall velocity, and the base velocity. Within the gap the fluid velocity profile is a linear combination of the Poiseuille and Couette velocity profiles.

Use Slip at walls when slip occurs at the wall and/or the base. In this case the difference between the wall or base velocity and the fluid velocity is proportional to the tangential part of the normal stress tensor component. The slip length divided by the fluid viscosity is the constant of proportionality in this relationship. The mean fluid velocity is computed using this assumption, given the pressure gradient and the wall and base velocities.

Enter a Slip length, wall Lsw. The default is 0.1 μm.

Select the Use different slip length for base check box to enter a Slip length, base Lsb. The default is 0.1 μm.

For Gas (modified Reynolds equation) it is possible to use the gas mean free path to specify the slip length. Change the Type of Slip setting (which defaults to Slip Length with the settings described above) to Mean free path and same accommodation coefficients or to Mean free path and different accommodation coefficients.

Define the Mean free path — Compute from material properties (the default), User defined expression, or User defined with reference pressure.

The Rarefied-total accommodation option provides a rarefied gas model that assumes total accommodation at the wall and the base. This model is accurate to within 5% over the range 0<Kn<880 (here Kn is the Knudsen number, which is the ratio of the mean free path to the film thickness). An empirical function, fitted to stationary solutions of the Boltzmann equation, is used to define the Poiseuille component of the flow.

Define the Mean free path — Compute from material properties (the default), User defined expression, or User defined with reference pressure.

Select the Force model:

The Rarefied-general accommodation option provides a rarefied gas model that assumes the same accommodation coefficient, α, at the wall and the base. This model is accurate to within 1% over the ranges 0.7<α<1 and 0.01<Kn<88 (here Kn is the Knudsen number, which is the ratio of the mean free path to the film thickness). An empirical function, fitted to stationary solutions of the Boltzmann equation, is used to define the Poiseuille component of the flow.

Define the Mean free path — Compute from material properties (the default), User defined expression, or User defined with reference pressure.

Select the Force model:

The User defined-relative flow function option enables user-defined models in which an effective fluid viscosity is employed. The fluid viscosity is divided by an additional factor Qch, which can be defined as an arbitrary expression. It is also possible to define the expressions for the fluid forces on the wall and base (these are included as feature inputs in other physics interfaces).

The User defined-general option allows for arbitrary flow models to be defined. Both the Poiseuille and Couette terms in the mean velocity can be defined arbitrarily. It is also possible to define the expressions for the fluid forces on the wall and base, (these are included as feature inputs in other physics interfaces).